Document transcript

Design and testing of a chopper amplifier used to amplify low-level signals.

Introduction

Several sensor outputs are DC signals in the microvolt to millivolt range. The DC amplifiers using opamps also have theinput offset in the same range. At DC frequency, the drift of the amplifier also affects the measurement. One of thetechniques used to achieve high precision dc gains with ac-coupled amplifiers is called chopper stabilization. In thistechnique, the input DC signal is modulated by a square wave, and an ac-coupled amplifier is used to amplify it. Further, itis synchronously demodulated and passed through a low pass filter to recover the amplified DC signal (Figure 1). Since theamplifier is ac-coupled, the input offset voltage does not appear at its output.

Figure 1: Principle of chopper amplifier

The Fourier representation of the chopping signal of unit amplitude,is given by the following equation:

Ifis the input signal to be measured, the Fourier representation of the modulated signal can be written as:

The modulation process translates the input signal to the odd harmonic frequencies of the chopping signal. The modulatedsignal is now amplified by an ac-coupled amplifier of gain A, and further demodulated back to the base band by thechopping signal. The output after demodulation can be represented by the following equation:

The output signal is recovered by passing the demodulated signal through a low pass filter which rejects all harmoniccontents.

The following figure (Figure 2) shows the entire process in frequency domain. The input signal is bandlimited to. Thechopper frequency

is chosen such that. The ac-coupled amplifier has a lower cut off frequency of

andthe low pass filter cut off

frequency is.

Figure 2: Principle of chopper amplifier in frequency domain

Multiplication of the input signal by a square wave is equivalent to switching the signal on and off. Therefore, a pair ofswitches, as shown in the following figure (Figure 3), can be used to realize the modulator and the demodulator. The twoswitches are driven by complementary control signals S and S'. The following circuit (Figure 4) consisting of twotransistors can be used to generate the control signals.

LowPassFilter

Output

Amp

Chopping Signal

t

v

10mV

t

v

10mV

0

t

v

0.5V

-0.5V

t

v

0.5V

t

v

0.23V

t

v

0

Indian Institute of Technology Kanpur

EE 381 Electrical Engineering Lab-II

2003–

04; Semester II

amountdoll_1ba0a1d8-f3ec-411e-ab32-372f6cb109d7.doc

2

of2

C1

C2

R1

R2

R4

R3

R5

-

+

+

-

Figure 3: Pair of switches used as modulator and demodulator

Figure 4: Control signal generator

Pre-laboratory exercise

Design the control signal generator circuit shown in Figure 3 meeting the following specifications.



The outputs S and S' are complement of each other.



The range of high (VoH) and low (VoL) levels are: 4V < VoH

< 5V and-5V < VoL

<-4V.



Choose the resistance values such that the transistors operate in either saturation or in cut off modes.



Simulate the circuit using PSPICE andcompare the simulated bias points with the design values.



Study the effect of driving the transistors too deep into saturation on the output of the circuit.



You will be using switching transistors 2N2369 in the laboratory to build this circuit. However, if you cannot find amodel for this transistor, use the model of 2N2222 transistor for the simulation.

Design an ac-coupled amplifier using the following schematic and meeting the following specifications:



Gain of the amplifier for the chopped signal is 100.



Explain the criteria for the choice of R1, C1, R2 and C2.



Using PSPICE simulate thefrequency response

of theamplifier.



Plot the simulated output

of the amplifier for a squarewave input of frequency 1kHz and having voltage levelsof 0V and 10mV.

Design a first order low pass filter to extract only the baseband signal at the output of the demodulator. Explain thecriteria used to choose the values of resistor and capacitor.

Set up the entire scheme of chopper amplifier on PSPICE.Use voltage controlled switch models for realizing themodulators if you cannot find a model for CD4066.

Set up a potential divider network to obtain voltages in the range of-50mV to +50mV. Use +5V and-5V power supply and10k pot to vary the voltage.

Set up a direct-coupled non-inverting amplifier using ua741 to give a DC gain of 100.Plot the gain

of the amplifier as afunction of input voltage over the range of-50mV to +50mV. Take sufficient number of points to demonstrate the effect ofDC offset as the input signal level approaches very small values. Do not use any offset nulling techniques to remove theDC offset.

Set up the control signal generator (Figure 2) and verify the functionality ofthe circuit.Plot the waveforms at both S andS'.

Set up the complete chopper amplifier circuit and for an input signal set to 10mV DC (which is set by the potential dividernetwork), note down the signals at theoutput of (a) modulator, (b) Amp1, (c) Amp2, (d) demodulator and (e) low passfilter.